Exactly what would happen to the Affordable Care Act if the Supreme Court invalidates tax credits in the three dozen states where the federal government runs the program?

Legal scholars say a decision like that would deal a potentially lethal blow to the law because it would undermine the government-run insurance marketplaces that are its backbone, as well as the mandate requiring most Americans to carry coverage.

The court in early 2015 is scheduled to hear King v. Burwell, a case that challenges the legality of tax credits in those states. The law's opponents argue that Congress intended to limit the subsidies to residents of states running their own health insurance exchanges. Currently, only 13 states and the District of Columbia fully operate exchanges on their own; another 10 are in some sort of partnership with the federal government. Federal officials operate the rest.​

Should the court find that subsidies in federally run exchanges are not allowed, "I don't think there are any rosy scenarios here," said Timothy Jost, a law professor at Washington and Lee University and a supporter of the health law. "It's a complete disaster."

The immediate impact is that the Internal Revenue Service would stop paying subsidies to those in federally run exchanges. In 2014, more than 4.6 million people were getting those subsidies. But by 2016, according to the Kaiser Family Foundation, the number could grow to as many as 13.4 million. (Kaiser Health News is an editorially independent program of the foundation.)

Most of those people losing subsidies would no longer be required by the "individual mandate" to have insurance, because they would fall into an exemption in the law for those who have to pay more than 8% of family income for health insurance premiums.

"Since a lot of people can't afford insurance without the tax credits, you're looking at a lot of people shedding coverage," says Nicholas Bagley, a law professor at the University of Michigan.

Those who hang onto their coverage and pay the entire premium without help "are likely to be sicker on average than the people who shed their coverage," says Bagley, "because they're the ones who need insurance the most."

Indeed, the insurance industry, through its trade group America's Health Insurance Plans, argued in a legal brief for a related case that the elimination of the federal exchange subsidies could seriously undermine those markets, creating an insurance death spiral.

"When healthy individuals opt out of the individual insurance market, those who are left are, on average, less healthy (and therefore prone to higher-than-average medical expenses)," AHIP said in its brief. "A sicker pool of consumers results in higher premiums, which causes an additional relatively healthy subset of participants to drop out, which in turn results in a further increase in premiums."

The impact also spreads beyond individuals who buy their own coverage, experts say. For example, eliminating subsidies for individuals also would eliminate the so-called "employer mandate" that seeks to require larger firms to provide coverage. That's because the employer mandate merely requires employers to pay a fine if their employees obtain subsidies on the exchange. If there are no subsidies, there are no employer fines and thus, effectively, no mandate.

Meanwhile, says Jost, "hospitals that have started to have some real relief from uncompensated care are right back taking care of uninsured people." In fact, he says, the problem of uninsured patients seeking hospital care could get worse because some people who had coverage in the old, unreformed individual market might have to drop it due to cost.

So what could be done? Clearly, states that are currently letting the federal government run their health exchange could establish their own marketplaces. But that's problematic as well, for reasons both practical and political.

"The practical obstacle is that creating an exchange is not child's play," says Bagley. "An exchange has to be a governmental entity or a non-profit entity. They've got to be able to carry out a variety of functions," including working with consumer assistance groups and overseeing compliance with the law's requirements.

While some have suggested that states could create a "virtual" exchange on paper and simply contract with the federal government to run it, Bagley says the law on the subject is pretty explicit. "States would have to do more than just the bare minimum," he said.

Timing and financing are other practical problems. The final deadline for states to apply for federal funding to establish an exchange has passed. And a decision from the Supreme Court is likely to come in late June of next year, which is after another deadline (June 15) for states to use their own funds to establish an exchange in time for the 2015-16 open enrollment season.

"What that means is that in many of these states that don't have exchanges, state legislatures will have to get involved," said Bagley. And many of those legislatures "are full of new members after the midterm elections who specifically campaigned against the ACA."

Still, some say the predictions of doom are overblown.

Should the court strike down subsidies in the federal exchanges, "I think we'll have some panic attacks and some exaggerated postures; people running around with their heads cut off for a couple of weeks," says health economist Tom Miller of the American Enterprise Institute.

But the main thing an anti-subsidy ruling would do, says Miller, is force Congress back onto the playing field to reopen the law.

"Congress will step in," he predicts. "We're going to have the kind of political give and take which was abbreviated and artificially truncated when the law was passed. It's not a pretty process, but that's why we have a government and we elect people."

"Money's not going to be ripped out of the hands of people that currently have coverage, at least for the second year of enrollment," he said. But eventually, "even President Obama will have to sign something that is more in accord with where the public is. No one gets their first choice."